The present invention relates generally to an intrusion monitor and more specifically to an improved intrusion monitor utilizing an infrared radiation detector capable of monitoring several physical areas at once in a simple and efficient manner.
Intrusion monitors of the prior art have a number of difficulties. Intrusion monitors are typically mounted on high on a wall and face a volume to be monitored. The monitors do not detect radiation from all volumes within a field of view, but only from a particular set of sub-volumes. This particular set is designed to permit the intrusion monitor to detect incident radiation from the selected sub-volumes which indicate an unauthorized entry into the protected volume. The set of sub-volumes does not typically include a dead zone sub-volume immediately under the monitor which extends from the wall to the first sub-volume which is monitored. Thus, a party desiring to gain entrance to a protected volume, could operate within the dead zone sub-volume without detection, effectively negating the intrusion monitor's purpose.
A prior art solution is to mount a mirror assembly external to a lens array which focuses incident radiation received from the monitored sub-volumes. The external mirror assembly is disposed to reflect radiation from dead zone sub-volumes into the lens array. Thus, radiation from the dead zone sub-volume is rerouted to enter the monitor where it may be detected. This solution has the disadvantage that its use alerts a would be unauthorized party to its intended purpose. That is, an external mirror assembly would tip off an unauthorized party that operation within previous dead zone sub-volumes would be detected.
The unauthorized party could then attempt an alternate method of entry or attempt to disable the external mirror assembly. As the external mirror assembly is accessible, disabling actions may be successful, and could be easily accomplished if there were periods of time in which the monitor was inoperative, such as during business hours. Additionally, external mirror assemblies would be affected by the environment and are generally more complex and thus expensive. Even such things as shipping and handling such a prior art monitor would be more expensive, as it is larger and bulkier, and more susceptible to accidental damage.
FIG. 1A is a graphical representation of a top view of a prior art intrusion detector 10. Detector 10 includes an infrared radiation sensor 12 protected from miscellaneous and extraneous infrared radiation by an envelope 14. Proximate to sensor 12 is a fresnel lens 16 to improve a range and sensitivity of sensor 12.
Fresnel lens 16 is a device well known in the art used to focus radiation onto sensor 12. The reader will understand the operation and uses of fresnel lenses, and no further description of their properties will be provided. Fresnel lens 16 is generally oriented to accept infrared radiation incident from a first field of view F.sub.1.
First field of view F.sub.1 may be narrow or relatively wide. Fresnel lens 16 has a defined relationship between it and sensor 12. A reference direction identified by datum line 20 defines a median direction from which incident radiation is effectively directed to sensor 12. A wide field of view refers to accepting radiation from within .THETA. degrees from datum line 20. Field of view F.sub.1 is then twice .THETA. or approximately 115.degree. in a plane including datum line 20 as depicted in FIG. 1A.
FIG. 1B is a perspective illustration of a side view of detector 10. Field of view F.sub.1 includes radiation received within .alpha. degrees of datum line 20. Thus, first field of view F.sub.1 is twice .alpha. or approximately 102.degree. in a plane containing datum line 20 and normal to the plane including .THETA..
FIG. 2A is a perspective illustration of a side view of detector 10' incorporating a fresnel lens array 30 in lieu of fresnel lens 16. Fresnel lens array 30 is comprised of a plurality of fresnel lenses 16.sub.i each having a particular field of view F.sub.i. The sum of the fields of view F.sub.i of each of fresnel lenses 16.sub.i make up a total field of view F.sub.T of lens array 30.
Each fresnel lens 16.sub.i of fresnel lens array 30 is oriented with its maximum sensitivity established in a different direction to improve total field of view F.sub.T for detectable radiation of fresnel lens array 30. It should be apparent that total field of view F.sub.T for fresnel lens array 30 in the vertical direction is greater than that of a fresnel lens 16. A typical fresnel lens array 30 has a total field of view F.sub.T range for .THETA. of approximately 110.degree..
FIGS. 2B and 2C are perspective illustrations of detector 10' during use as an infrared intrusion detector, with FIG. 2B illustrating a side view and FIG. 2C illustrating a top view. Detector 10' is typically mounted relatively high on a surface 40, e.g., a wall, and inclined approximately 10.degree. to 14.degree.. Detector 10' faces a volume V to be monitored. Fields of view F.sub.i of the individual fresnel lenses 16.sub.i define a pattern of discrete sub-volumes (V.sub.11 -V.sub.mn) of volume V to be monitored. The pattern of these discrete sub-volumes V.sub.ii is designed to maximize protection of the entire volume V from intrusion.
However, because fresnel lens array 30 has a limited field of view, there are "blind spots," or dead zone sub-volumes Z, which cannot be monitored by detector 10'. This dead zone Z generally extends from wall 40 a significant distance. Dead zone Z extends approximately 10 feet for detector 10' mounted about 7 feet high and inclined about 12.degree. from the horizontal. An intruder in this area would be able to advance or operate without detection by detector 10'.
U.S. Pat. No. 4,752,769, issued June 21, 1988 to Knaup et al., discloses a mirror assembly mounted exterior of a fresnel lens array to permit radiation outside a field of view of the lens array to be focused into the lens array.